Setup reverse proxy with nginx for ddos mitigation

I need nginx loaded and setup as a reverse proxy and tweaked for ddos mitigation so filtered traffic can be passed to my real back-end server. Features should include a small PHP script that can be activated using the nginx .conf.d file that when activated presents presents users with a challenge page the says "Click here to enter site". Once clicked their IP is white-listed in IP tables for X amount of minutes and then flushed.